The electric cable to which the invention applies is a cable for conveying a signal, in particular at high or very high frequency. Within a coaxial cable, it serves to convey a signal picked up by an antenna, in particular a WiFi, Bluetooth, GPRS, Mimo, 3G, etc., antenna incorporated in an electronic or computer appliance.
The specifications laid down by electronic, computer, and telecommunications industries concerning such cables are tight, thereby requiring very precise shapes and materials to be used, thus making it very difficult to fabricate cables that satisfy all of the requirements laid down.
In known manner, it is possible to make an electric cable comprising a conductor in an insulating sheath, by forming a solid sheath around the conductor, in particular forming a sheath made out of polyethylene (PE), polytetrafluoroethylene (PTFE), or indeed fluorinated ethylene propylene (FEP). These materials have the advantage of presenting a dielectric constant (ε) that is very small.
In spite of that, when making such cables, the dielectric properties (dielectric constant ε, dissipation factor tan δ) are not satisfactory; or indeed the required level of mechanical strength is not reached. In particular, it is difficult to achieve values for the dielectric constant ε that are less than 2, and in particular less than 1.7.
Patent EP 0 803 878 presents an alternative solution for making such cables. That document proposes making the sheath out of a material having a low dielectric constant, such as FEP or PE.
It also provides, more particularly, for the presence of longitudinally-extending cells within the thickness of the sheath of the conductor, thus making it possible to reduce the dielectric constant of the material.
Nevertheless, that document does not describe fabricating cables using PTFE and at least one conductor in an insulating sheath. That document proposes a solution only for fabricating a conductor that is sheathed in a thermoplastic material. Unfortunately, known thermoplastic materials possess dielectric properties (dielectric constant ε, factor tan δ) that are not sufficient, given the required dielectric properties, even when the above-mentioned cells are formed in the wall of the sheath on the conductor. Furthermore, using such conductors requires special precautions to be taken when making connections by soldering, since the materials tend to melt under the effect of one or more heater elements, and that is a phenomenon that it is naturally desirable to avoid.
In addition, that document does not make provision for the possibility of the cable having a plurality of conductors.